Long-term adaptation of dark fermentation from food waste to saline marine organic matter

The growth of marine-based industries has stimulated interest in developing biotechnological pathways to valorize marine organic matter. Dark fermentation is a potential conversion path; however, research on its efficacy under saline conditions is limited. This study investigates the transition of a terrestrial dark fermentation system processing food waste into a saline dark fermentation system processing marine organic matter through 130-day semi-continuous trials. A four-stage transition was adopted for a gradual change from food waste to marine organic matter. The average volatile fatty acids (VFAs) yield from the initial food waste stage was 6.7 g/L at below 8 parts per thousand (ppt) salinity. The highest VFA concentration (12.3 g/L) was observed at 30 ppt salinity. High prominence of acetic acid (>75%) was observed throughout all stages. The transition to marine organic matter, a lower carbon-nitrogen substrate than food waste, resulted in a 3.6-fold increase in total ammoniacal nitrogen, reaching 668 mg/L. Chemical oxygen demand (COD) balance and chemical energy distribution were used to approximate a specific hydrogen yield of the marine organic matter at 19.4 mL H₂/g COD. Microbial communities were evaluated at different stages of salinity, highlighting a shift from lactic-acid-associated taxa (Bifidobacterium and Lactobacillus spp.) to more diverse communities dominated by the Clostridia and Thermatogae classes, in line with increased acetic acid yields under saline conditions. Saline dark fermentation offers a viable pathway to convert marine organic matter into VFAs and allows for integration with biotechnologies, such as microbial fuel cells.